Magnesium-based alloys



United tare are This invention relates to low density alloys having ahigh modulus of elasticity and, more particularly, to magnesium-basealloys and a method of producing said alloys.

'Magnesium alloys and other low density alloys have been the chiefstructural materials for applications requiring high strength-to-weightratios in their construction materials. This has been especially thecase in the aircraft and other transportation vehicles and additionallyin many consumer items such as household goods, ofiice equipment,instruments and sporting goods.

. In the selection of structural materials, however, other factors thanstrength-to-weight ratios must be considered. For example, theresistance to elastic deformation of the structural material must beconsidered. The resistance of a material to elastic deformation is knownas stiffness and represents the extent of the elastic deformations ordefiections which take place under given stresses. The most desirablestructural material should not only be capable of withstanding highstresses, but should also exhibit relatively little deformation underthese stresses. This resistance to elastic deformations is measured bythe value of the modulus of elasticity of the material and is a separatefunction of the yield point or ultimate strength of the material. Theactual deformations or deflections produced in stressed members dependon the modulus of elasticity of the material and the geometry of themember. It is often necessary, especially in structures made ofmagnesium, to design the members on the basis of their stiffness ratherthan on their strength. Many members are designed with largercross-sections than are required to carry the given stresses because thelarger cross-section is needed to give the structure the requiredstiffness. Because manylo'w-density materials, such as magnesium and itsalloys, have a comparatively low-modulus of elasticity, the savings inWeight that could be realized by the use of these materials is oftenoffset by the need to design and use larger sections to provide thenecessary stiffness.

. The modulus of elasticity of pure magnesium is about 65x10 pounds pervsquare inch while the modulus of elasticity of steel is about x10 poundsper square inch. It .is easily seen-how important a factor thelowmodulus of magnesium has. been in preventing the full employment ofthis metals valuable properties. Another factor to be considered in theselection of structural materials is the coefficient of expansion of thematerial. An excessively high rate of expansion can limit the usefulnessof the material .and a low coefiicient of expansion isthereforedesirable. Unfortunately many lowdensity alloys have highcoefficients of expansion.

It is the primary object. of this invention, therefore, to

' provide a magnesium-base alloy having a higher modulus ofelasticityuthan pure magnesium and heretofore produced magnesium-basealloys.v 2

It is alsoyan object of this invention to provide a magnesium-base alloyhaving a high ratio of modulus, of

' elasticity to-density.

It is a further object of this invention to provide a magnesium-basealloy having a lower coefiicient of expansion than pure magnesium andmany heretofore produced magnesium alloys. I

It is a further object of this invention to provide a 5 method forproducing the'magnesium-base alloy described herein.

' Other aims and objects of this invention will be ap- 3,lh,4 l5Patented Jan. 19, 19fi5 parent from the following description andappended claims.

In accordance with these objects a method for producing a high modulusmagnesium-base alloy is provided which comprises dispersing finelydivided particles of a refractory metal boride in a magnesium matrix.The process of the invention comprises preparing a mixture consistingessentially of from about 10 to about 50 volume percent of particles ofborides of at least one metal selected from the group consisting oftitanium, chromium, zirconium, tantalum, columbium, hafnium, vanadiumand molybdenum, and the balance powdered magnesium material selectedfrom the group consisting of magnesium and magnesium-base alloys,compacting the resulting mixture into a coherent slug, heating theresulting slug in the absence of air to a temperature at which themagnesium material melts without substantial vaporization and below themelting point of the metal borides, and allowing the slug to cool. In apreferred embodiment the slug is cooled to a temperature below thefreezing point of the mag nesium material and worked into a desiredshape.

In the practice of the process of this invention an alloy is producedconsisting essentially of from about 10 to about 50 volume percent ofparticles of borides of at least one metal selected from the groupconsisting of titanium, chromium, zirconium, tantalum, columbium,hafnium, vanadium, and molybdenum, and the balance substantially all ofa metal selected from the group consisting of mag nesium and magnesiumalloys, said particles of metal borides being dispersed in a continuousmatrix of the magnesium material.

Specifically an alloy is provided consisting essentially of from about10 to 50 volume percent of titanium diboride dispersed in a continuousmatrix 'of magnesium or a magnesium-base alloy. l Q

A particularly useful matrix material is a magnesiumaluminum alloyconsisting of about 5 percent by Weight aluminum and the balancemagnesium. This alloy was used as the matrix material in most of theexamples fol- I lowing. In making the alloys of this invention the mag-I nesium-base alloy used as the matrix material is reported the matrixis defined in volume percents because the volume percent actuallysuggests the amount of dispersed phase in a cross-section of the matrix.Since the densities of the borides and the matrix of materials differ,it would I not be suggestive of the true amount of dispersed phasepresent if it were reported in wei ht percents;

Asummary of the properties of the alloy of this invention, as comparedto the properties of a commercial magnesium alloy is presented inTable 1. As may be seen from the table, the specific modulus, or ratioof modulus of elasticity-to-density, of the alloy of the invention,containing as little as 10 percent by volume ofadispers'ed phase oftitanium diboride in a magnesium-5 percent aluminum alloy is increasedby more than 13 percent. The resulting modulus of elasticity of thisalloy is greater than that of one of the best commercial magnesiumalloys as listed in Table 1. When the amount of the dispersed titaniumdiboride phase'is increased to 40 percent by volume of the alloy, theincrease in specific modulus over the best 0.5 percent zinc, 0.15percent manganese and the balance magnesium.

The dispersed phase must be not 3 TABLE I 10 vol. 20 vol. 40 vol.percent; percent percent TiBz in T1132 in Til in Magne- Magne- Magne-Comsium-5 slum-5 slum-5 mereial weight weight Weight alloy percentpercent percent aluminum aluminum aluminum alloy alloy alloy TensileStrength:

Ultimate, 75 F- 47, 200 5-5, 000 44, 425 53, 000 Yield, 75 F 41, 000 40,000 Ultimate, 750 3,090 3, 550 3, 600 Yield, 750 12. 2, 810 3, 000Elongation, percent in 75 F 7. 5 5 7 F 11 12 3. 7 Reduction Area,percent:

y p.s.i.X10 75 F..-" 8.3 11 17.6 6. 5 Density, g/cc 2.02 2. 3 2.85 1. 8Specific Modulus (Mod. of ElasJ Density) 4. 11 4. 7 6. 17 3. 6 ThermalOoeflicient of Linear Expansion, C. 10 24-427 C. 28. 32 2l.o2 19. 59 29.8 Condition containing as high as 50 volume percent titanium diborideshow an increase in elastic modulus. It is to be noted that the modulusof elasticity of a volume percent titanium diboride magnesium alloy isalmostdouble that of pure magnesium. It is also to be noted-that thestrength of the 20 volume percent titanium diboride-aluminum alloycompares favorably with the commercial alloy; It has also been foundthat when titanium diboride or other metal borides are dispersed inhigh-strength commercial magneinsoluble in the matrix, but it must alsobe subject to an intimate bond with the matrix under the varyingtemperature and pressure conditions of the metalworking operation. Theseconditions are particularly well met by the combination of a titaniumdiboride phase dispersed in a magnesium or magnesium alloy'matrix. Themagnesium material completely wets the titanium diboride and yet showsno tendency to react chemically with it. The unique character of themixture is further illustrated by the fact that when an unsupported slugof the compacted powders, in which the low melting magnesium matrixoccupies as much as 40 percent of the volume, is heated above themelting point of the magnesium, and maintained in that condition, theslug, although apparently molten, retains,

its original shape and does not slump into a formless pool of moltenmagnesium. This eitect, believed to be caused by a strong tendency ofthe magnesium to wet the diboride phase, is also believed to beresponsible for the excellent degree of dispersion of the diborideparticles in the magnesium matrix. Since heating is carried out in avacuum or inert atmosphere, the fact that no gross melting occurs cannotbe attributed to the presence of oxide or oxide coating acting as,a'skin to hold the shape of the article.

The most uniform dispersions of the titanium diboride phase in amagnesium matrix have been obtained by the use of a new techniquecombining compaction and con trolled melting.

In this process, .the'powders of the diboride and the 7 magnesiummaterial are dry blended and cold compacted at a pressure of from about30,000 to about 50,000 pounds per square inch into a slug of convenientdimensions. The

magnesium is the powder. produced by spraying molten magnesium andcooling it rapidly to form a powder. The

slug is heated, in a vacuum or in aninert atmosphere, to

about 800 .C. which is about 225 C. in excess of the meltingpoint ofmagnesium. Gross melting does not occur, probably as the result ofcapillarity dueto the wetting of'thetitanium diboride particles by themolten sium alloys such as that listed in Table La superior high- Ymodulus alloy results.

Another featureof the alloy of this invention is its low coefiicient oflinear expansion in'comparison with-comsion of pure magnesium.

TABLE II and only about tworthirds the normal coefiicient of expan-Ooelfieient of Expansion per Material degree 0.,

magnesium material. I, g

While ordinary powder metallurgical techniques, such as cold pressingand sintering, or hot pressing can be used to produce articles of thisalloy they do not produce the same superioralloy product asthecompaction-controlled 10 vol. percent TlBziD. magnesium-5% Al alloy- 28.32 20 vol. percent TiBg in magnesium-5% A1 alloy 21.32 40 vol. percentTiBg in magnesium-5% Al alloy. 19. 59 Commercial alloy 8 In ordertoassure that the desired eiiect is permanent, it

is necessary that the constituentsbe mutually nonreactive and insolublein each other. The diborides of tantalum, Zirconium, chromium,columbium, hafnium, vanadium and molybdenum are allrelatively insolublean'd nonreac- V tive in magnesium materials. 4 only unreactive andmelting technique described herein. Although to all out; wardappearancesthe constitution of the alloy is the same,

the two products have different properties; We're identical mixtures oftitanium diboride and magnesium powders to be subjected to the samefabricating procedures, except .that hotpressingis-substituted for thecompaction-controlled meltingtechnique, the modulus of elasticity-of theproducts would differ.. The 'sintered or hot pressed prod;

ilct would. have a modulus much less than that of the 7 product producedby the process of this invention.

Since a dense, strong material is produced after the .firinggoperationit is practical to lose the product in the as-fired condition, or tofabricate the as-fired composition by rolling, for'ging orswagingwithout the necessity of first extruding the product. Y V r Whenthe firedproductis extruded, it canibe fabricated into useful shapes byconventional working techniques. V

ypracticeof the invention.

The-following example. is presented to illustrate the Y ExampleThirty-nine percent by weight ('20 volume percent) of dry, finelydivided titanium diboride was blended with 61 percentby weight volumepercent) of a mixture containingS percent by weight of atomiged aluminumand the balance atomized magnesium and the mixture was .cold compactedat"40-,000 pJSjplntO l- /z' inch diam eter slug approximately 2, incheslong. The slug was heated under vacuum conditions to 800 C. and heldthere for about minutes. Gross melting did not occur but rather the slugretained its shape while the molten magnesium-aluminum alloy flowedcompletely in and around each particle of titanium diboride producingthereby the desired dispersion. After this heat treatment, the slug wascooled below the point at which the magnesiumaluminum alloy solidified,and removed to an extrusion press where it was reduced from a 1 /2 inchingot to a rod inch in diameter by the application of about 60,000 psi.The density of the rod is the as-extruded condition was about 2.3 g./cc. and it exhibited a Youngs modulus of elasticity at 75 F. of about11X 10 p.s.i. as determined sonic means. The coefiicient of linearexpansion in the range of to 427 C. was found to be 2l.32 10 These andother properties are contained in Table 1. The other alloys noted inTable 1 were made in the same manner as above.

In another example 52.5 percent by weight titanium diboride volumepercent) was mixed with 47.5 percent by weight powdered magnesium metal(70 volume percent). This mixture was cold compacted into a coherentslug. The slug was heated under vacuum conditions at about 770 C. Therewas no gross melting but rather the magnesium metal melted and flowedaround the diboride particles to form the desired matrix. The fired slugwas then extruded into a desired shape at around 480 C.

It is to be noted that while the description of the inven tion has beenin terms of magnesium and magnesiumaluminum alloys as matrix materials,that other magnesium materials such as any of the many commercialmagnesium-base alloys are included. It is only necessary that themagnesium-base alloy used be low melting, ductile and light weight andthat it be mutually non-reactive and insoluble with the borides used inthe dispersed phase and that the magnesium-base alloy have the propertyof flowing around and wetting the dispersed particles when the compactedslug of these materials are heated above the melting point of themagnesium-base alloy.

What is claimed is:

1. The method of preparing a magnesium-base alloy product characteridedby an increased modulus of elasticity and a decreased coefiicient ofthermal expansion which comprises preparing a mixture consistingessentially of from about 10 to about 50 volume percent of particles ofborides of at least one metal selected from i the group consisting oftitanium, chromium, zirconium,

tantalum, columbium, hafnium, vanadium and molybdenum, and the balance apowdered magnesium material selected from the group consisting ofmagnesium and magnesium-base alloys, compacting the resulting mixtureinto a coherent slug, heating the resulting slug in the absence of airat a temperature at which the magnesium material melts withoutsubstantial vaporization and below the melting temperature of the metalborides, cooling the slug to a temperature below the freezing tempera-:ture of the selected magnesium material, and forming the slug into adesired shape.

v 2. The method of preparing a magnesium-base alloy productcharacterized by an increased modulus .of elasticity and a decreasedcoefiicient of thermal expansion which comprises preparing a mixtureconsisting essentially of from about 10 to about 50 volume percent ofparticles of borides of at least one metal selected from the groupconsisting of titanium, chromium, zirconium, tantalum, columbium,hafnium, vanadium and molybdenum, and the balance a powdered magnesiummaterial selected from the group consisting of magnesium andmagnesiumbase alloys, compacting the resulting mixture into a coherentslug having a desired shape, heating the resulting slug in the absenceof air at a temperature at which the magnesium material melts withoutsubstantial vaporization and below the melting temperature of the metalborides, and cooling the slug.

3. The method of preparing a magnesium-base alloy product characterizedby an increased modulus of elasticity and a decreased coefficient ofthermal expansion which comprises preparing a mixture consistingessentially of from about 10 to about 50 volume percent of particles ofborides of at least one metal selected from the group consisting oftitanium, chromium, zirconium, tantalum, columbium, hafnium, vanadiumand molybdenum and the balance a powdered magnesium material selectedfrom the group consisting of magnesium and magnesiumbase alloys,compacting the resulting mixture at a pressure of from about 30,000 to50,000 pounds per square inch into a slug, heating the resulting slug inthe absence of air at a temperature between the melting temperature ofthe selected magnesium material and about 800 C. cooling the slug to atemperature below the freezing temperature of the selected magnesiummaterial, and forming the slug into a desired shape.

4. The method of preparing a magnesium-base alloy product characterizedby an increased modulus of elasticity and a decreased coefiicient ofthermal expansion which comprises preparing a mixture consistingessentially of from about 10 to about 50 volume percent of particles oftitanium diboride, and the balance substantially all powdered magnesium,compacting the resulting mixture into a coherent slug, heating theresulting slug in the absence of air at a temperature at which themagnesium melts without substantial vaporization and below the meltingtemperature of the metal borides, cooling the slug to a temperaturebelow the freezing temperature of the magnesium, and forming the slugintoa desired shape.

5. The method of preparing a magnesium-base alloy product characterizedby an increased modulus of elasticity and a decreased coeflicien-t ofthermal expansion which comprises preparing a mixture consistingessentially of from about 10 to about 50 volume percent of particles oftitanium diboride, and the balance a powdered magnesium materialcontaining 5 percent by weight aluminum and the balance substantiallyall magnesium, compacting the resulting mixture into a coherent slughaving a desired shape, heating theresulting slug in the absence of airat a temperature at which the magnesium material melts withoutsubstantial vaporization and below the melting temperature of the metalboride for at least a time sufiicient for the magnesium material tomelt, cooling the slug to a temperature below the freezing temperatureof the magnesium material, and forming the slug into a desired shape.

References Cited in the file of this patent Edward Arnold Ltd, 1960, pp.627-628. (Originally published in Iron Steel Inst. Special Report, No.58, 1956, pp. 242-248, by MacDonald and Ransley.)

Progress in Powder Metallurgy, Capital City Press, volume 16, 1960, pp.99-119.

1. THE METHOD OF PREPARING A MAGNESIUM-BASE ALLOY PRODUCT CHARACTERIZEDBY AN INCREASED MODULUS OF ELASTICITY AND A DECREASED COEFFICIENT OFTHERMAL EXPANSION WHICH COMPRISES PREPARING A MIXTURE CONSISTINGESSENTIALLY OF FROM ABOUT 10 TO ABOUT 50 VOLUME PERCENT OF PARTICLES OFBXXXXXX OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OFTITANIUM, CHROMIUM, ZIRCONIUM, TANTALUM, COLUMBIUM, XXXNIUM, VANADIUMAND MOLYBDENUM, AND THE BALANCE A POWDERED MAGNESIUM MATERIAL SELECTEDFROM THE GROUP CONSISTING OF MAGNESIUM AND MAGNESIUM-BASE ALLOYS,COMPACTING THE RESULTING MIXTURE INTO A COHEREENT SLUG, HEATING THERESULTING SLUG IN THE ABSENCE OF AIR AT A TEMPERATURE AT WHICH THEMAGNESIUM MATERIAL MELTS WITHOUT SUBSTANTIAL VAPORIZATION AND BELOW THEMELTING TEMPERATURE OF THE METAL BORIDES, COOLING THE SLUG TO ATEMPERATURE BELOW THE FREEZING TEMPERATURE OF THE SELECTED MAGNESIUMMATERIAL, AND FORMING THE SLUG INTO A DESIRED SHAPE.